Renege in Cribbage is when you should play a card but do not. For example, if you say 'Go' when you could have played a card without going over 31. In Cribbage you must always play a card if you can, so reneging is against the rules.
The ACC Cribbage Rules state that (in ACC-sanctioned tournament play), a player may correct a "go" call before either player pegs a point or before the opponent plays a card. If this doesn't happen and the player subsequently plays a renege card, the opponent may claim a renege up to the time he or she plays the next card or announces the count of his or her hand.
When a renege is claimed, any cards played after the renege or "go" are returned to the players, and any points pegged (or scored) are cancelled. The player who was reneged against gets two points for each renege card: for example, if the count is 25 and a player holds A, 5, and 6 and does not play any of the three cards and says "go," that is a "triple renege," and the opponent gets six points (two for each card). See penalties in cribbage for details of other penalty points that can be scored in cribbage.